A thermoelectric heat exchanger is a device based on the thermocouple. An elementary thermocouple comprises an electrical circuit including two dissimilar conductors connected in series. When a direct current (DC) is passed through the circuit, heat is pumped from one of the electrical connections between the dissimilar conductors to the other, giving rise to a hot junction and a cold junction. The converse effect also occurs, i.e. heating one junction and cooling the other will generate a flow of DC in the electrical circuit. Reversing the direction of DC flow interchanges the hot and cold junctions.
The effect of an elementary thermocouple is small and a practical thermoelectric heat exchanger assembly requires a large number of elementary thermocouples to be electrically connected in series. This gives rise to an alternating series of hot junctions and cold junctions. A physical structure is thus required to put each cold junction in thermal contact with a cold fluid and likewise to put each hot junction in thermal contact with a hot fluid. These structures are referred to respectively as a hot exchanger and as a cold exchanger. In both cases thermal contact must be achieved by the exchangers without electrically shorting the series connection of elementary thermocouples.
Generally speaking, the dissimilar conductors which give rise to the thermoelectric effect are constituted by suitable alternating P type and N type semiconductor material, e.g. bismuth telluride, and are referred to herein as P type thermoelements and N type thermoelements. P type thermoelements transfer heat in the same direction as the flow of electric current and N type thermoelements transfer heat in the opposite direction. The thermoelements are electrically interconnected by heat conductors which thereby constitute the hot or cold junctions and form parts of the respective exchangers. Although the heat conductors thus form third (and possibly fourth) dissimilar types of conductor in the series connection of thermocouples, their contribution to, or interference with, the thermoelectric effect is minimal.
The above outline recalls the well-known essentials of thermoelectric heat exchanger assembly. The present invention applies more particularly but not exclusively to the type of thermoelectric heat exchanger which keeps the heat conductors at different temperatures, the thermoelements being fed with DC to maintain a temperature difference betwwen the heat conductors. Such a device is called a "heat pump" and can also be used for air conditioning by heating or cooling a fluid by means of heat in the ambient atmosphere.
It has already been proposed to manufacture thermoelectric heat exchanger assemblies which include thermopiles with hot and cold heat exchangers which are electrically conductive and separated alternately by P type and N type thermoelements, the electric current flowing in the pile direction, e.g. vertically. These vertical thermopiles are disposed side by side so as to constitute an assembly with a parallelepipedical appearance in which the small plates or pellets which constitute the thermoelements are disposed in a succession of horizontal planes and the heat exchangers are disposed in intermediate horizontal planes. One of the heat exchanger fluids flows horizontally through each heat exchanger, the cold fluid flowing in one direction and the hot fluid flowing in the other.
The mechanical structure is designed to conciliate the sealing requirements for the fluid flow circuits with differential thermal expansion. It is particularly tricky to produce such a structure since known bismuth telluride based thermoelements are extremely fragile.
Such a structure is described for example in U.S. Pat. No. 3,626,704 (Coe).
It suffers from several drawbacks of which two are particularly important when at least one of the hot and cold fluids is a gas, say air. The first of these drawbacks lies in the problem of passing enough air through a gas heat exchanger, i.e. one extracting heat from or giving it up to a gas. This problem leads to the provision of large heat exchangers giving rise to bulky apparatus as a whole. The second of these drawbacks lies in the fact that the gas duct follows a zig-zag path when passing through the successive gas heat exchangers of a single thermopile. This leads to considerable head loss, and hence to a requirement for bulky and/or noisy fans.
Another prior document French Pat. No. 2,035,167 discloses apparatus likewise comprising a thermopile in the form of a vertical stack or stacks of the abovementioned type, i.e. each stage of the thermopile comprises a P type thermoelement (for example), a hot exchange (for example), a, N type thermoelement and a cold exchanger, with current flowing in series through the said four components of a stage and through all the stages of the thermopile. The thermopile is kept permanently under vertical compression by a spring acting on one of its ends, thereby avoiding the appearance of mechanical tensions in the thermoelements. This apparatus transfers heat between a duct of hot air and a duct of cold air arranged on either side of the thermopile. Each heat exchanger includes a first portion disposed inside the thermopile, held tightly between two thermoelements and conducting the electric current, and a second portion outside the thermopile in the corresponding air duct to exchange heat with air therein. Heat is thus transferred sideways between the thermopile and each of the ducts. The apparatus has a hot side and a cold side and is therefore subject to assymetrical thermal contractions and expansions which tend to curve the thermopile. This tends, in the end, to cause the thermoelements to crack and even to break.